X-ray high voltage devices are configured to control the voltage supplied to an X-ray tube. For example, an X-ray high voltage device is configured to control output voltage by switching on and off switching elements included in an inverter circuit. When the switching process is performed at a higher frequency, advantages are achieved where output voltage ripples are reduced and where the structure (coils, a transformer, capacitors, and the like) of the X-ray high voltage device can be kept compact. However, because the number of times the switching elements are switched on and off increases, losses that occur during the switching process (hereinafter, “switching losses”) increase. In this regard, for the purpose of reducing such switching losses, it is possible to make use of soft switching control such as Zero Voltage Switching (ZVS) control and Zero Current Switching (ZCS) control.
The zero voltage switching control is a controlling method implemented to reduce switching losses, by which the voltage is caused to resonate as a result of storing and extracting an electric charge into and from a capacitor (a resonant capacitor) provided in parallel to a switching element, so that the switching process is performed at the time when the voltage of the switching element becomes substantially 0. In this situation, what extracts the electric charge from the resonant capacitor is an inverter current flowing through an inverter circuit. Thus, when the inverter current is small, it may be impossible, in some situations, to extract the electric charge from the resonant capacitor until the voltage becomes substantially 0. For example, when the output power of the X-ray high voltage device is small or the like, it may be impossible to implement the zero voltage switching control while the load is small, in some situations. In other words, even when an attempt is made to implement the zero voltage switching control while the load is small, it is impossible to extract the electric charge from the resonant capacitor until the voltage becomes substantially 0. As a result, because the switching element is turned on while the voltage is not substantially 0, a switching loss occurs. Further, when the switching element is turned on while the voltage is not substantially 0, a short-circuit current may flow into the switching element due to the electric charge stored in the resonant capacitor and may damage the switching elements in some situations.
In contrast, the zero current switching control is a controlling method implemented to reduce switching losses, by which a switching process is performed at a time when the inverter current becomes substantially 0. In this situation, the X-ray high voltage device includes a high voltage transformer, mold resin, and the like, which substantially function as a capacitor. The electrostatic capacitance (a stray capacitance) of such a capacitor may cause the inverter current to have a current resonance. Thus, when the switching element is turned on while the electric current (hereinafter, “current”) is not substantially 0, a switching loss occurs. Further, when the output power of the X-ray high voltage device is large or the like, it may be impossible, in some situations, to implement the zero current switching control while the load is large, due to the absence of the time at which the current becomes substantially 0.